Splitting water with semiconducting photoelectrodes—Efficiency considerations

Weber, M.; Dignam, M. J.

doi:10.1016/0360-3199(86)90183-7

Cited by 126 publications

(96 citation statements)

References 9 publications

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“…Photocatalytic water splitting using a 2-photon device has the potential for much higher efficiency than a 1-photon device. 3,4 In a 2-photon device the optimal bandgaps for the 2 photoabsorbers are approximately 1.7 eV and 1.0 eV. [3][4][5] This allows for the large bandgap material to absorb the blue light leaving the red light to be absorbed by the small bandgap material.…”

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confidence: 99%

Mo₃S₄Clusters as an Effective H₂Evolution Catalyst on Protected Si Photocathodes

Seger

Herbst

Pedersen

et al. 2014

J. Electrochem. Soc.

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This work shows how a molecular Mo 3 S 4 cluster bonded to a photoelectrode surface via a phosphonate ligand can be a highly effective co-catalyst in photocathodic hydrogen evolution systems. Using a TiO 2 protected n + p Si photocathode, H 2 evolution occurs with an onset of +0.33 V vs. RHE in an acid solution with this precious metal-free system. Using just the red part of the AM1.5 solar spectrum (λ > 635 nm), a saturation current of 20 mA/cm 2 is achieved from an electrode containing Mo 3 S 4 dropcasted onto a 100 nm TiO 2 /7 nm Ti/n + p Si electrode. It is well established that society cannot indefinitely rely on fossil fuels and must switch to a renewable energy source.1 Solar fuels such as hydrocarbons from CO 2 reduction and H 2 from the water splitting reaction are highly attractive since these molecular products can be stored indefinitely 2 and used directly in the transportation infrastructure. Photocatalytic water splitting using a 2-photon device has the potential for much higher efficiency than a 1-photon device. 3,4In a 2-photon device the optimal bandgaps for the 2 photoabsorbers are approximately 1.7 eV and 1.0 eV. [3][4][5] This allows for the large bandgap material to absorb the blue light leaving the red light to be absorbed by the small bandgap material. Silicon is very promising as the small bandgap material since it has a bandgap of 1.1 eV. A major problem with Si and other small bandgap materials is that they have stability problems in a water splitting environment. Recently it was shown that depositing a thin film of TiO 2 mitigates stability issues.6-8 However, the rectifying properties of this film may restrict the ability to electrodeposit catalysts onto the protected photoabsorber surface.9,10 Even though the TiO 2 protection layer can be detrimental to certain electrodeposition procedures, it may also open up a new avenue to help improve the stability of molecular catalysts.Previous work showed molecular clusters such as Mo 3 S 4 incomplete cubane clusters to be quite effective as H 2 evolution catalysts, 11,12 however the materials are also known to be water soluble. This solubility makes it quite difficult to keep these catalysts adsorbed on an electrode surface in a working environment. A previous approach to resolve this issue involved attaching hydrophobic ligands to the clusters. 13,14 Unfortunately this approach decreased catalytic activity and stability in air. One of the great difficulties in depositing these catalysts was the limited freedom in interacting with the Si surface due to its extreme propensity to oxidize to SiO 2 . However with the emergence of TiO 2 as a protection layer, there now is a much larger freedom to design molecular catalysts that can interact with the electrode.In the present work we investigated attaching a (hydrophilic) molecular linker to help bind Mo 3 S 4 clusters to the TiO 2 surface. Previous work showed that phosphonate groups are quite effective at binding to TiO 2 in photocatalytic H 2 evolving systems.15 Thus a N-(phosphonomethyl)iminodia...

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confidence: 99%

Mo₃S₄Clusters as an Effective H₂Evolution Catalyst on Protected Si Photocathodes

Seger

Herbst

Pedersen

et al. 2014

J. Electrochem. Soc.

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“…Direct solarto-fuels conversion can be achieved by interfacing suitable catalysts that carry out two separate half-reactions of water splittingthe four electron-four proton oxidation of water to O 2 and the two electron two-proton reduction of the produced protons to H 2 -to a photovoltaic (PV) material. Numerous device configurations have been proposed for photoelectrochemical (PEC) water splitting (4)(5)(6)(7)(8)(9)(10), and they can be broadly categorized into those devices wherein the photovoltaic material makes a rectifying junction with solution as opposed to those in which the rectifying junctions are protected from solution or "buried." Fig.…”

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Interplay of oxygen-evolution kinetics and photovoltaic power curves on the construction of artificial leaves

Surendranath

Bediako

Nocera

2012

Proc. Natl. Acad. Sci. U.S.A.

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An artificial leaf can perform direct solar-to-fuels conversion. The construction of an efficient artificial leaf or other photovoltaic (PV)-photoelectrochemical device requires that the power curve of the PV material and load curve of water splitting, composed of the catalyst Tafel behavior and cell resistances, be well-matched near the thermodynamic potential for water splitting. For such a condition, we show here that the current density-voltage characteristic of the catalyst is a key determinant of the solar-to-fuels efficiency (SFE). Oxidic Co and Ni borate (Co-B i and Ni-B i ) thin films electrodeposited from solution yield oxygen-evolving catalysts with Tafel slopes of 52 mV∕decade and 30 mV∕decade, respectively. The consequence of the disparate Tafel behavior on the SFE is modeled using the idealized behavior of a triple-junction Si PV cell. For PV cells exhibiting similar solar power-conversion efficiencies, those displaying low open circuit voltages are better matched to catalysts with low Tafel slopes and high exchange current densities. In contrast, PV cells possessing high open circuit voltages are largely insensitive to the catalyst's current density-voltage characteristics but sacrifice overall SFE because of less efficient utilization of the solar spectrum. The analysis presented herein highlights the importance of matching the electrochemical load of water-splitting to the onset of maximum current of the PV component, drawing a clear link between the kinetic profile of the water-splitting catalyst and the SFE efficiency of devices such as the artificial leaf.energy storage | solar energy | solar fuel | buried junction W ater splitting is the central chemistry that underlies the storage of solar energy in the form of chemical fuels because it delivers hydrogen from a renewable source (1-3). Direct solarto-fuels conversion can be achieved by interfacing suitable catalysts that carry out two separate half-reactions of water splittingthe four electron-four proton oxidation of water to O 2 and the two electron two-proton reduction of the produced protons to H 2 -to a photovoltaic (PV) material. Numerous device configurations have been proposed for photoelectrochemical (PEC) water splitting (4-10), and they can be broadly categorized into those devices wherein the photovoltaic material makes a rectifying junction with solution as opposed to those in which the rectifying junctions are protected from solution or "buried." Fig. 1 schematically depicts the latter with a double-junction configuration of absorber materials with progressively larger band gaps that are connected in series using thin-film ohmic contacts to generate open circuit voltages (V oc ) that are large enough to drive water splitting. Thin-film ohmic contacts at the termini of this stack serve both to protect the semiconductor from the chemistries occurring in solution and to enable efficient charge transfer to catalyst overlayers, which execute the oxygen-evolution reaction (OER) and hydrogen-evolution reaction (HER). Whereas waterspli...

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“…The specific case of water splitting is covered in both older 5,6 and more recent literature. [7][8][9][10] Depending on the exact assumptions related to losses, the optimum band gaps for the top-and bottom cells in the tandem are in the range of 1.7 eV -1.9 eV and 0.95 eV -1.4 eV respectively; 4 and the corresponding maximum achievable solar-to-hydrogen (STH) efficiency 11 is in the range of 20% -29%.…”

Section: Photoelectrochemical Water Splittingmentioning

confidence: 99%

Performance Limits of Photoelectrochemical CO₂ Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products

Vesborg

Seger

2016

Chem. Mater.

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Solar-driven reduction of CO 2 to solar fuels as an alternative to H 2 via water splitting is an intriguing proposition. We model the solar-to-fuel (STF) efficiencies using realistic parameters based on recently reported CO 2 reduction catalysts with a high performance tandem photoabsorber structure. CO and formate, which are both two-electron reduction products, offer STF efficiencies (20.0% and 18.8%) competitively close to that of solar H 2 (21.8%) despite markedly worse reduction catalysis. The slightly lower efficiency toward carbon products is mainly due to electrolyte resistance, not overpotential. Using a cell design where electrolyte resistance is minimized makes formate the preferred product from an efficiency standpoint (reaching 22.7% STF efficiency). On the other hand, going beyond a 2 electron reduction reaction, the more highly reduced products seem unviable with presently available electrocatalysts due to excessive overpotentials and poor selectivity. This work considers breaking up the multielectron reduction pathway into individually optimized, separate twoelectron steps as a way forward.

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Splitting water with semiconducting photoelectrodes—Efficiency considerations

Cited by 126 publications

References 9 publications

Mo₃S₄Clusters as an Effective H₂Evolution Catalyst on Protected Si Photocathodes

Mo₃S₄Clusters as an Effective H₂Evolution Catalyst on Protected Si Photocathodes

Interplay of oxygen-evolution kinetics and photovoltaic power curves on the construction of artificial leaves

Performance Limits of Photoelectrochemical CO₂ Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products

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Splitting water with semiconducting photoelectrodes—Efficiency considerations

Cited by 126 publications

References 9 publications

Mo3S4Clusters as an Effective H2Evolution Catalyst on Protected Si Photocathodes

Mo3S4Clusters as an Effective H2Evolution Catalyst on Protected Si Photocathodes

Interplay of oxygen-evolution kinetics and photovoltaic power curves on the construction of artificial leaves

Performance Limits of Photoelectrochemical CO2 Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products

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Mo₃S₄Clusters as an Effective H₂Evolution Catalyst on Protected Si Photocathodes

Mo₃S₄Clusters as an Effective H₂Evolution Catalyst on Protected Si Photocathodes

Performance Limits of Photoelectrochemical CO₂ Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products